Poly(vinyl trifluoroacetate) has commercial importance as a precursor to highly syndiotactic poly(vinyl alcohol) hydrogels, which themselves are commercially important as materials for, e.g., contact lenses, membranes, and as coatings for polymeric and non-polymeric substrates such as membranes. Heretofore, the polymerization of vinyl trifluoroacetate has posed a number of significant problems.
Poly(vinyl trifluoroacetate) with high molecular weight has been prepared by free radical bulk polymerization (U.S. Pat. No. 2,436,144) but the process is not workable on a large scale due to the exothermic character of the reaction. The reaction mixture sets to a gel at low conversion because the polymer is insoluble in, but swollen by, the monomer.
Polymerization of vinyl trifluoroacetate in common organic solvents has been described. Haas et al., Journal of Polymer Science, Vol. 22, p. 291 (1956) describe a reaction in acetone resulting in a low molecular weight material. Pritchard et al., Journal of Polymer Science, Part A-1, Vol. 4, p. 707 (1966) describe synthesis in n-heptane at 80.degree. C. with low conversion and in carbon tetrachloride at -50.degree. C., also with low conversion. Harris et al., Journal of Polymer Science, Part A-1, Vol. 4 p. 665 (1966) describe a reaction in n-heptane at -78.degree. C. to give a high molecular weight polymer. Such low temperature reaction conditions are difficult to maintain on a commercial scale.
Polymerization of vinyl trifluoroacetate in inert chlorofluorocarbon solvents such as that available under the trademark FREON 113 has been described by Gagnon, et al. in pending U.S. patent application Ser. No. 07/005,969, filed Nov. 8, 1991, assigned to the assignee of the present invention, which is a continuation-in-part of the following U.S. patent application Ser. Nos.: 07/605,754; 07/606,757; 07/605,828; 07/605,834; 07/605,921; and 07/605,948; all filed Oct. 30, 1990. A similar synthesis was described by Hammar et al. in U.S. Pat. No. 4,673,539, assigned to the assignee of the present invention, with the exception that Hammar et al. did not use trifluoroacetic anhydride as a water scavenger. Commercial use of such chlorofluorocarbon solvents is severely restricted for environmental reasons.
Polymerization of vinyl trifluoroacetate in siloxanes and perfluorinated solvents has been described by Eian in pending U.S. patent application Ser. No. 08/152,393, assigned to the assignee of the present invention, filed Nov. 12, 1993. While use of these solvents allows preparation of poly(vinyl trifluoroacetate) on a commercial scale, these solvents are expensive. Also, the siloxanes are flammable.
The starting monomer (vinyl trifluoroacetate) for poly(vinyl trifluoroacetate) is extremely moisture sensitive. Polymerization of vinyl trifluoroacetate by aqueous emulsion or suspension techniques is not possible because hydrolysis of the monomer competes with polymerization of the monomer. Even small amounts of water must be avoided because acetaldehyde, formed as a hydrolysis product, acts as an inhibitor of the chain polymerization reaction.
Supercritical fluids have been used as extraction and chemical reaction media because of the range of properties such as density, diffusivity, viscosity, cohesive energy density, surface tension, etc. which can be adjusted by simply changing the pressure [L. Boock, et al., CHEMTECH, Vol. 22, p. 719 (1992)]. Generally the supercritical fluids are advantageously used to selectively solubilize certain materials in the presence of others, to dissolve materials that are otherwise difficult to dissolve, and to provide a simple and efficient means of isolating solubilized material when pressure is released.
Fukui, et al. (U.S. Pat. No. 3,522,228) describe a method for polymerizing a vinyl compound in the presence of a carbon dioxide medium. This description includes supercritical and non-supercritical reaction conditions for CO.sub.2. Although Fukui, et al. describe the reaction of a variety of vinyl compounds in liquid carbon dioxide, there is no reference to the polymerization of any moisture sensitive monomers such as vinyl trifluoroacetate under supercritical or non-supercritical conditions nor is there any detailed reference to the polymerization of fluorinated monomers other than passing reference to tetrafluoroethylene (which is not a moisture sensitive monomer). Knowing the difficulty in achieving poly(vinyl trifluoroacetate) of reasonable molecular weight, one skilled in the art would assume that the process described by Fukui et al. would not be suitable for preparing this polymer.
Desimone, et al. (Science, Vol. 257, p. 945 (1992) and International Publication No. WO 93/20116(14.10.93) discuss the homogeneous free radical polymerization of highly fluorinated vinyl monomers in supercritical carbon dioxide. The polymerization described by Desimone proceeds because the reaction mixture (i.e. monomer, polymer and initiator) remains homogenous. Desimone teaches that the reaction mixture needs to remain homogeneous during the entire course of the polymerization to obtain a polymer of reasonable molecular weight.
U.S. Pat. No. 5,312,882 discloses a heterogeneous emulsion or suspension polymerization of water insoluble monomer in CO.sub.2 in the presence of a surfactant. The patent describes polymerization of monomers in a discontinuous phase where CO.sub.2 is the continuous phase.